Download International Electrical Engineering Journal (IEEJ) Vol. 6 (2015) No.6, pp. 1930-1936

International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1930-1936
ISSN 2078-2365
http://www.ieejournal.com/
Grid Connection and Islanding
Operation of Distributed Generation
with Synchronous Frame Controller
L.Vamsi Narasimha Rao , S.N.V.S.K.Chaitanya
Department of Electrical & Electronics Engineering, V.R.Siddhartha Engineering College,
Vijayawada, Krishna (Dist), A.P, India.
[email protected], [email protected]
Abstract— Renewable energy resources (RES) are being
increasingly connected in distribution systems utilizing power
electronic converters. This paper presents a novel control
strategy for the synchronous frame control also called DQ
control, uses reference frame transformation module eg: abc to
dq to transform the grid current and grid voltage waveforms
into a reference frame that rotates synchronously with grid
voltage. By means of this, the control variable becomes dc
values then filtering and controlling can be achieved. This paper
also deals with islanding operation which activates when the
voltage and frequency of the grid exceeds the specified value at
the point of common coupling. Thus potency of the technique is
illustrated using MATLAB Simulation.
Index Terms— Phase Locked Loop, Photo voltaic cell, PEM
fuel cell, Synchronous frame controller, MATLAB/Simulation
I. INTRODUCTION
Now a day’s using of the power electronics devices poses a
Power quality problems and the concept of micro grid is
useful (helpful) to consumer for reliable and efficient power
Supply and reduce [1] the per unit cost of the electrical load
And reduce the total energy losses and also the
interconnection of micro grid to the utility grid with the help
Of power electronics devices poses power quality problems.
The power quality problems like frequency deviation and
load current in the micro grid. A number of control strategies
have been developed by researchers but choice of control
strategy is important to cope with the operating condition of
system. In this paper five control strategies viz. instantaneous
p-q theory, synchronous reference frame Method (SRF)[2],
instantaneous symmetrical component theory (ISCT) and
Average unit power factor theory(AUPFT). This paper
represents the micro source devices are connected to the
utility grid through dc to dc converters and voltage source
inverters synchronous reference frame Method (SRF) at the
Point of Common Coupling (PCC). The proposed micro grid
consists of a photovoltaic array, proton exchange membrane
Fuel cell. The photovoltaic array operates main generation
unit of the micro grid. During the sunless hours proton
exchange membrane fuel cell operates the main generation
unit. As such modern control theories are used to implement
the system such controllers present in this paper as
Synchronous frame control (SRF) theory or DQ theory.
II.PHOTO-VOLTAIC SYSTEM
The Solar cells are the medium to convert solar energy into
the electrical power. These cells are made up of semiconductor
materials, when sun beam is absorbed with these material
electrons emits and releases the current and thus electric power
is produced.
The power generated by a single module may not be
sufficient to supply the most of the appliances. So a group of
modules are connected in series which is generally used for
high voltage applications and in the same way they are
connected in parallel, the connection which is useful for high
current applications.
MODELING OF PV SYSTEM
The single diode model of a single PV cell [3] is
manifested in the figure 1. It includes a current source, a
diode, parallel connected to the current source which
represents the photocurrent, a series resistance Rs and a
parallel resistance Rsh. An accurate single diode model is
depicted in the above figure. Equation (1) represents the
current generated from the cell
𝐼𝑝𝑣 = 𝐼𝑝ℎ 𝑁𝑝 − 𝐼𝑠 𝑁𝑝 [exp (
(
𝑁
𝑝
𝑞(𝑉𝑝𝑣 +𝐼𝑝𝑣 𝑅𝑠 𝑁 𝑠 )
𝑁
𝑅𝑃 𝑠
𝑁
𝑝
𝑞(𝑉𝑝𝑣 +𝐼𝑝𝑣 𝑅𝑠 𝑁 𝑠 )
)
𝑎𝑉𝑡 𝑁𝑠
) − 1] −
(1)
𝑁𝑝
1931
Narasimha and Chaitanya
Grid Connection and Islanding Operation of Distributed Generation with Synchronous Frame Controller
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1930-1936
ISSN 2078-2365
http://www.ieejournal.com/
written as follows,
𝐼𝑝𝑣 = 𝐼𝑝ℎ 𝑁𝑝 − 𝐼𝑠 𝑁𝑝 [exp (
𝑎𝑉𝑡 𝑁𝑠
𝑁𝑠
)
𝑁𝑝
) − 1]
𝑁
𝑞 (𝑉𝑝𝑣 + 𝐼𝑝𝑣 𝑅𝑠 𝑠 )
𝑁𝑝
−(
𝑁
𝑅𝑃 𝑠
𝑁𝑝
Fig 1. Single diode model of PV cell
Where
III. PROTON EXCHANGE MEMBRANE FUEL CELL
I0 is the diode’s reverse saturation current
VT is the diode’s thermal voltage
a is the ideality factor of the diode
The equation of a PV current as a concomitant of changing
environmental conditions, the temperature and irradiance can
be written as
𝐼𝑝ℎ = [𝐼𝑠𝑐 + 𝐾1 (𝑇𝑐 − 𝑇𝑟𝑒𝑓 )]𝑆/1000
𝐼𝑠 = 𝐼𝑟𝑠 (𝑇𝑐 /𝑇𝑟𝑒𝑓 )3 exp[𝑞𝐸𝑔 (
1
1
𝑇𝑟𝑒𝑓
− )/𝐾𝐴 ]
𝑇𝑐
Many papers have researched in the mechanism and
experience models of the fuel cells. Different modeling
methods have different complexities according to the number
of parameters that may be discussed. We also can see the
develop history from these papers. All in all, the most
specific experience model we can refer to is
The output voltage of the single cell is given by (6)According
to the PEMFC [5] output characteristics empirical equation,
(2)
Where
IPV_STC is the photocurrent under Standard Test Conditions
(STC)
ΔT=T-TSTC (in Kelvin) and TSTC=25°C
G is the irradiance on the surface of the cell
GSTC is the irradiance under STC (1000W/m²)
KI is the short circuit current coefficient (generally provided
by the manufacturer)
The equation for the saturation current of the diode is given as
(3)
Where
Eg is the energy gap of the semiconductor
I0_STC is the nominal saturation current
The reverse saturation current equation can be further
improved as a function of temperature as follows
𝐼0 =
𝑞 (𝑉𝑝𝑣 + 𝐼𝑝𝑣 𝑅𝑠
𝑉𝑐𝑒𝑙𝑙 =𝐸𝑁𝑒𝑟𝑛𝑠𝑡 -𝑉𝑎𝑐𝑡 -𝑉𝑜ℎ𝑚𝑖𝑐 -𝑉𝑜ℎ𝑚𝑖𝑐
𝐸𝑁𝑒𝑟𝑛𝑠𝑡 =
1
2𝐹
(6)
(∆𝐺 − ∆𝑆(𝑇 − 𝑇𝑟𝑒𝑓 ) + 𝑅𝑇 (𝑙𝑛𝑃𝐻2 +
𝑙𝑛𝑃𝑂2
𝑉𝑎𝑐𝑡 =𝜉1 +𝜉2 T+𝜉3 T[ln(𝐶𝑂2 )]+ 𝜉4 T[ln(i)]
(8)
In this model, all the parameters are obtained experimentally,
and are listed in Table I.
TABLE I
Parameters Of The Pemfc Dynamic Model
Value
Parameters
Value
N
20
𝜉1
-0.9514
T(K)
323
𝜉2
0.00312
𝑃𝐻2
0.5
𝜉3
7.4*10^-5
KV is the temperature coefficient of open circuit
𝑃𝑂2
0.5
𝜉4
-1.87*10
ISC_STC is the nominal short circuit current
VOC_STC is the nominal open circuit voltage
∆𝐺
237180
l(µm)
51
A(cm2)
150
𝜆
20
∆𝑆(𝑚𝑜𝑙 )
-163.15
B(V)
0.016
𝑇𝑟𝑒𝑓
298.15
C(F)
2.5
F(C/mol)
96486.7
Jmax(A/cm2)
1.5
R(J/mol K)
8.314
Rc(Ώ2)
3*10^-4
(4)
exp[(𝑉𝑜𝑐 +𝐾𝑉∆𝑇)𝑎𝑉𝑇 ]−1
Where
voltage
PV ARRAY MODELING
All the above equations are applicable for a single PV cell.
But in a typical installation of a PV [4] power station, PV
modules are used in which series and parallel connected PV
cells are used in order to bridge the supply demand gap.
Series combination of the cells increases the voltage and the
parallel combination of the cells increases the current of the
entire module .In such case, the output equation can be
))
The activation loss of PEMFC is caused by the sluggish
kinetics of the reactions taking place on the active surface of
electrodes and it can be computed by the following equations
Parameters
𝐼𝑠𝑐 +𝐾𝐼 ∆𝑇
2
(7)
1932
Narasimha and Chaitanya
Grid Connection and Islanding Operation of Distributed Generation with Synchronous Frame Controller
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1930-1936
ISSN 2078-2365
http://www.ieejournal.com/
𝐶𝑂2 =
𝑃 𝑂2
5.08∗106𝑒𝑥𝑝
(9)
498
(− 𝑇 )
Different from the ohmic losses of the empirical equation that
is already presented, in this model, ohmic losses consist of
the voltage drop that is caused by RM , the equivalent
membrane impedance, and the voltage drop that is caused by
Rc , the contact resistances both between the membrane and
electrodes as well as the electrodes and the bipolar plates.
It is constant once the cell is fabricated. It can be shown as
Vohmic =iR ohmic =i (Rm+RC)
(10)
The equivalent membrane impedance RM can be expressed
via Ohms law
r
RM= ml
(11)
A
The resistivity 𝑟𝑚 of a Na-f ion series proton exchange
membrane can be calculated by [18]
𝑟𝑚 =
181.6[1+0.03(𝑖/𝐴)+0.062(𝑇/303)2(𝑖/𝐴)2.5]
𝑖
𝐴
[𝜆−0.634−3( )]exp[4.18{
(12)
𝑇−303
}]
𝑇
Where λ is the water content of the membrane,
which is an adjustable parameter and a function of the
relative humidity of the gas in anode, and has a Value
In this model, the effect of concentration losses is also
considered which is different from previous models
.Concentration losses are caused by mass transportation,
which, in turn, affects the concentration of the hydrogen and
oxygen at high current density.
This term is ignored in some models, perhaps because it is not
desirable to operate the stack at regions where concentration
losses are high (efficiency is low). However, if the stack
operates at high current density, this term needs to be
included.
The concentration losses can be expressed as
𝑉𝑐𝑜𝑛 =B ln (1-
𝐽
𝐽𝑚𝑎𝑥
)
(13)
Fig.2 Electrical model of PEMFC
Circuit model of the PEMFC in which an electrical capacitor
can be considered as the layer of charge on or near the
electrode–electrolyte interface, which is a store of electrical
charge and energy, as shown in Fig. 4.In Fig. 4, Ra is the
equivalent resistance that includes the activation equivalent
resistance Ract and the concentration equivalent resistance
Rcon; the equivalent capacitance C can effectively smooth the
voltage drop across Ra . As a function of the charge double
layer, the PEMFC bears eminent dynamic characteristics. If
νd is the overall voltage drops across Ra.
IV. BOOST CONVERTER
The output voltage of solar cell and PEM fuel cell is
connected to boost converter in order to boost up the output
voltage.
The boost converter is a medium of power transmission to
perform energy absorption and injection from DG to grid-tied
inverter. The process of energy absorption and injection in
boost converter is performed by a combination of four
components which are inductor, electronic switch, diode and
output capacitor. The connection of a boost converter is
shown in Figure 2 [6]. The process of energy absorption and
injection will constitute a switching cycle . In other word, the
average output voltage is controlled by the switching on and
off time duration. At constant switching frequency, adjusting
the on and off duration of the switch is called
pulse-width-modulation (PWM) switching. The switching
duty cycle, k is defined as the ratio of the on duration to the
switching time period. The energy absorption and injection
with the relative length of switching period will operate the
converter in two different modes known as continuous
conduction mode (CCM) and discontinuous conduction
mode (DCM) [4][6].
Fig.3 Schematic of boost converter
V.ISLANDING ISSUES
Although there are some benefits of islanding
operation there are some drawbacks as well. Some of them
are as follows: Line worker safety can be threatened by DG
sources feeding a system after primary sources have been
opened and tagged out. The voltage and frequency may not
be maintained within a standard permissible level. Islanded
system [7] may be inadequately grounded by the DG
interconnection. Instantaneous reclosing could result in out
of phase reclosing of DG. As a result of which large
mechanical torques and currents are created that can damage
the generators or prime movers .Also, transients are created,
which are potentially damaging to utility and other customer
equipment. Out of phase reclosing, if occurs at a voltage
peak, will generate a very severe capacitive switching
transient and in a lightly damped system, the crest
over-voltage can approach three times rated voltage. Various
1933
Narasimha and Chaitanya
Grid Connection and Islanding Operation of Distributed Generation with Synchronous Frame Controller
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1930-1936
ISSN 2078-2365
http://www.ieejournal.com/
risks resulting from this include the degradation of the
electric components as a consequence of voltage& frequency
drifts.
VI.PHASE-LOCKED LOOP
A phase-locked loop (PLL) is a control system that
generates an output signal whose phase is linked to
the phase of an input signal. A phase locked loop is used to
determine the frequency and angle reference of the Point of
Common Coupling (PCC) voltage.
Grid synchronization is achieved relying on the grid
voltage angle that is computed by a PLL. Since an accurate
and fast detection of the grid voltage angle allows to inject
current with a unitary power factor the PLL has a remarkable
influence on the quality of the energy generated by DGs.The
proposed control strategy, a single phase PLL structure is
derived starting from the three phases PLL [8] scheme based
on the synchronous reference frame to detect voltage angle.
This topology relies on the synchronism between the grid
voltage vector and the synchronous reference frame ,it can be
adopted for a single-phase system providing that a quadrature
voltage is suitably generated. Grid-connected operation
consists in delivering power to the local loads and to the
utility grid. In such a case, the output voltage reference is
often taken from the grid voltage sensing, by using a
phase-locked-loop (PLL) circuit, while control system
ensures that the inverter acts as a current source .
Now, the two phase current quantities iα and iβ of stationary
αβ-axes are transformed into two-phase synchronous (or
rotating) frame (d-q-axes) using equation (15), where cosθ
and sinθ represents the synchronous unit vectors which can
be generated using phase-locked loop system (PLL).
𝐢𝐝
𝐜𝐨𝐬𝛉 𝐬𝐢𝐧 𝛉 𝐢𝛂
[𝐢 ]=[
][ ]
−𝐬𝐢𝐧 𝛉 𝐜𝐨𝐬𝛉 𝐢𝛃
𝐪
(15)
VI.SYNCHRONOUS FRAME CONTROLLER
There are different control strategies being used for the
calculation of reference currents in active power filter namely
Instantaneous Reactive Power Theory (p-q theory), Unity
Power Factor method, One Cycle Control, Fast Fourier
Technique etc. Here, SRF theory is used to extract the
three-phase reference currents (ica*, icb*,icc*) used by the
active power filters [9]. Figure 2 shows the block diagram
which explains SRF-theory. This theory is based on the
transformation of currents in synchronously rotating d-q
frame. Voltage signals are processed by the PLL to generate
the unit vectors. Current signals are transformed into d-q
frame and then filtered.
Then compensating current transformed back to a-b-c
frame and fed to hysteresis current controller for switching
pulse generation. In this method, the source currents (ia ,ib,
ic) are first detected and transformed into two-phase
stationary frame (αβ-0) from the three-phase stationary frame
(a-b-c), as per equation (14).
𝟏
𝐢𝛂
𝟐
[𝐢𝛃 ]=√𝟑 𝟎
𝐢𝟎
𝟏
[𝟐
−𝟏
𝟐
√𝟑
𝟐
𝟏
−𝟏
𝟐
−√𝟑
𝟐
𝟏
√𝟐
√𝟐
]
𝐢𝐚
[𝐢𝐛 ]
𝐢𝐜
(14)
Fig 4. A General structure for synchronous rotating d-q reference frame
control.
The d-q currents thus obtained comprises of AC and DC
parts. The fundamental component of current is represented
by the fixed DC part and the AC part represents the harmonic
component. This harmonic component can be easily
extracted using a high pass filter (HPF), as implemented in
Figure 2. The d-axis current is a combination of active
fundamental current (id dc) and the load harmonic current
(ih). The fundamental component of current rotates in
synchronism with the rotating frame and thus can be
considered as dc. By filtering id, the current is obtained,
which represents the fundamental component of the load
current in the synchronous frame. Thus, the AC component id
can be obtained by subtracting id dc part from the total d-axis
current (id), which leaves behind the harmonic component
present in the load current. In the rotating frame the q-axis
current (iq) represents the sum of the fundamental reactive
load currents and part of the load harmonic currents. So the
q-axis current can be totally used to calculate the reference
compensation currents. Now inverse transformation[10] is
performed to transform the currents from two – phase
1934
Narasimha and Chaitanya
Grid Connection and Islanding Operation of Distributed Generation with Synchronous Frame Controller
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1930-1936
ISSN 2078-2365
http://www.ieejournal.com/
synchronous frame d-q into two-phase stationary frame α-β
as per equation (16).
𝐢𝛂 𝐜𝐨𝐬𝛉
[𝐢 ]=[
𝐬𝐢𝐧 𝛉
𝛃
−𝐬𝐢𝐧 𝛉 𝐢𝐝
][ ]
𝐜𝐨𝐬𝛉 𝐢𝐪
(16)
Grid connected Test DG system is in fig 5.The system is
operated in grid connected mode and Islanding mode.
The grid voltage and current waveforms is depicted in
Fig.6. The inverter is operated to deliver for the load of 1kW.
Finally the current from two phase stationary frame αβ0 is
transformed back into three-phase stationary frame abc and
the compensation reference currents ica*, icb* and icc* are
obtained is shown in(18)
𝐢𝛂
𝐢∗𝐜𝐚
∗
[𝐢𝐜𝐛 ] = [𝐓𝐚𝐛𝐜 ] [𝐢𝛃 ]
𝐢∗𝐜𝐚
𝐢𝟎
(17)
Fig 6.Grid Voltage and Current Waveform
where
𝟏
𝟐 −𝟏
𝟐
−𝟏
Tabc=√𝟑
[𝟐
Hence
𝐢∗𝐜𝐚
[𝐢∗𝐜𝐛 ]
𝐢∗𝐜𝐚
𝟏
=
𝟐 −𝟏
√ 𝟐
𝟑
−𝟏
[𝟐
𝟏
𝟎
√𝟑
𝟐
−√𝟑
√𝟐
𝟏
√𝟐]
𝟐
𝟎
√𝟑
𝟐
−√𝟑
𝟐
For these different conditions synchronous controller
performance is observed. When any fault or any fault in
system it goes under islanding mode of operation.
The Islanding condition is performed to observe the
synchronous controller performance and the output voltage
and output current are observed.
√𝟐
𝟏
𝟏
√𝟐
𝟏
√𝟐
𝟏
√𝟐]
𝐢𝛂
[𝐢𝛃 ]
𝐢𝟎
(18)
The equation (18) represents the final observation for abc to
dq0 transformation ,and the generated pulses fed to inverter
Through gating pulses using PWM Technique as discussed in
the above fig 4.
VII.SIMULATION MODELS &RESULTS
The proposed system is simulated in MATLAB/SIMULINK
using Sim power systems tool boxes. The overall simulated
diagram for grid connected PV&PEMFC based DG system is
mentioned below.
Fig7. Islanding Mode Of Operation
The output voltage and current waveforms of islanding
condition is observed in the below Fig 8.
Fig.8.Load Output Voltage And Current Waveforms
Fig 5. Grid connected Test DG system
By the detection logic the fault or swell is detected
with help of IEEE 2003 standard, that is when the voltage,
frequency exceed this specified standard circuit breaker trips
which acts as a Reclosure.
1935
Narasimha and Chaitanya
Grid Connection and Islanding Operation of Distributed Generation with Synchronous Frame Controller
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1930-1936
ISSN 2078-2365
http://www.ieejournal.com/
The circuit breaker is connected to the PCC for which
synchronization algorithm [11] works for islanding
operation, the simulated model is shown in fig 9.
The islanding detection mode is mentioned is mentioned
below
meteorological uncertainty) in reliably coordinating different
sources with the utility. In future the multi agent applications
maybe developed that provides intelligence to this developed
model. The multi agent systems are used to control
distributed generation system in a simulated environment
during various fault conditions
REFERENCES
Fig 9.Islanding Detection System
The islanding detection can be done through a logic in which
it takes the line voltage as basic data, while the fault occurs
voltage swells will takes place and then this logic will send
signals to circuit breaker and circuit will trip the line.
Fig 10. Grid voltage under fault condition
After the fault is cleared the inverter voltage is synchronized
[12] with grid. The voltage at fault condition is shown in fig
14.Thus the paper describes about grid voltage to be operated
in the NDZ regions.
Fig 11.Grid voltage when detected
Once synchronization with the grid is completed, the DG
was reconnected to the grid that is from islanding mode of
operation to grid connected mode operation.
VIII.CONCLUSIONS
In this paper, the modeled hybrid distributed generation
system is devised to accomplish the task of supplying
uninterrupted power to meet the load demands with
intentional islanding algorithm. The control strategy is
observed, In addition to the benefits mentioned, control
strategy addresses the immense bottlenecks (arising due to
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1936
Narasimha and Chaitanya
Grid Connection and Islanding Operation of Distributed Generation with Synchronous Frame Controller